The long-term objectives of the study are to characterize sites on the surface of mammalian sperm which interact with the egg to insure fertilization. The exposure of these sites during epididymal maturation, their modification following ejaculation, their possible involvement in the acrosome reaction, as well as the species specificity of binding have important implications in the understanding of the basic features of gamete interaction. The manipulation of such sites may be the basis for novel methods of fertility regulation. Mouse in vitro fertilization offers a system with well defined steps for the study of gamete interactions. Murine sperm need to be capacitated to bind to the zona, they then undergo the acrosome reaction, penetrate the zona and fuse with the vitellus. A 83,000 mw glycoprotein, ZP3, from the zona has been implicated in both the binding and the induction of the acrosome reaction. What appears to be the complimentary site on the sperm, at least for zona binding, has been recently isolated and partially characterized. This acceptor site on epididymal sperm also binds a protein inhibitor of seminal vesicle origin. The specific aims of this proposal are to characterize this site and to determine the extent to which it participates in sperm-zona binding. Specific experiments are designed to determine (1) if solubilized zonae from two cell embryos and unfertilized eggs compete with the seminal inhibitor for the acceptor site on capacitated and uncapacitated sperm, (2) if purified acceptor can be demonstrated on the zonae of treated eggs and (3) the molecular weight of the component(s) of the zona to which the acceptor will attach. Furthermore, with antibodies which are already made to the acceptor, it will be possible to correlate the appearance of the site with its functional capabilities during the epididymal sojourn and in vitro and in utero incubation. Additionally, the modifications which occur in the acceptor site, as they relate to inhibitor binding will be investigated, using radio-labeled seminal inhibitor, by the classical receptor-agonist methodology. These studies should provide valuable, basic information concerning the characteristics of this site, as well as to define its role in sperm-zona interactions.